How to teach your horse the extended trot?

I'm always a big fan of poles for helping everything along. Set up two sets: one at your average stride and another at a larger stride. When you go towards the large stride set, ask for your extended trot. Your horse will connect the cue with the result when they have to take that stride to make it between the poles

I've always had luck by sitting deep and squeezing their sides until I've felt a difference in stride but not just going faster once I've gotten the littlest amount of change I release and trot a lap and then do it again